On the Kazakh-Uzbek border, surrounded by miles of toxic desert, lies an island. Or at least, something that used to be an island.

Vozrozhdeniya was once home to a vibrant fishing village fringed by turquoise lagoons, back when the Aral Sea was the fourth-largest in the world and abundant with fish.

But after years of abuse by the Soviets, the waters have receded and the sea has turned to dust; the rivers that fed it were diverted to irrigate cotton fields. Today, a layer of salty sand, riddled with carcinogenic pesticides, is all that remains of the ancient oasis.

This is a place where the mercury regularly hits 60C (140F) in the sandy soil, and where the only signs of life are the skeletons of desiccated trees and camels shading under giant, stranded boats.

Now Vozrozhdeniya has swallowed up so much of the sea that it’s swelled to 10 times its original size, and is connected to the mainland by a peninsula. But it is thanks to another Soviet project that it is one of the deadliest places on the planet.

From the 1970s, the island has been implicated in a number of sinister incidents. In 1971, a young scientist fell ill after a research vessel, the Lev Berg, strayed into a brownish haze. Days later, she was diagnosed with smallpox. Mysteriously, she had already been vaccinated against the disease. Though she recovered, the outbreak went on to infect a further nine people back in her hometown, three of whom died. One of these was her younger brother.

A year later, the corpses of two missing fishermen were found nearby, drifting in their boat. It’s thought that they had caught the plague. Not long afterwards, locals started landing whole nets of dead fish. No one knows why. Then in May 1988, 50,000 saiga antelope which had been grazing on a nearby steppe dropped dead – in the space of an hour.

The island’s secrets have endured, partly because it isn’t the kind of place where you can just turn up. Since Vozrozhdeniya was abandoned in the 1990s, there have only been a handful of expeditions. Nick Middleton, a journalist and geographer from Oxford University, filmed a documentary there back in 2005. “I was aware of what went on, so we got hold of a guy who used to work for the British military and he came to give the crew a briefing about the sorts of things we might find,” he says.

“He scared the pants off me, to be honest.”

That expert was Dave Butler, who ended up going with them. “There was a lot that could have gone wrong,” he says. As a precaution, Butler put the entire team on antibiotics, starting the week before. As a matter of necessity, they wore gas masks with hi-tech air filters, thick rubber boots and full white forensic-style suits, from the moment they arrived.

They weren’t being paranoid. Aerial photographs taken by the CIA in 1962 revealed that while other islands had piers and fish-packing huts, this one had a rifle range, barracks and parade ground. But that wasn’t even the half of it. There were also research buildings, animal pens and an open-air testing site. The island had been turned into a military base of the most dangerous kind: it was a bioweapons testing facility.

The project was a total secret, not even marked on Soviet maps, but those in the know called it Aralsk-7. Over the years the site flourished into a living nightmare, where anthrax, smallpox and the plague hung in great clouds over the land, and exotic diseases such as tularemia, brucellosis, and typhus rained down and seeped into the sandy soil.

Aralsk-7 was part of a bioweapons program on an industrial scale

The island was isolated enough that it wasn’t discovered until the 19th Century, making it the perfect place to hide from the prying eyes of Western intelligence. Failing that, the surrounding sea made a convenient natural moat.

These are the factors that led to it being chosen as the final resting place for the largest anthrax stockpile in human history. Its origins remain obscure, but it’s possible that the deadly cache was manufactured at Compound 19, a facility near the Russian city of Sverdlovsk, now Yekatarinburg.

Aralsk-7 was part of a bioweapons program on an industrial scale, one that employed over 50,000 people at 52 production facilities across the Soviet empire. Anthrax was produced in huge fermenting vats, tenderly nurtured as though they were growing beer.

In 1988, nine years after an anthrax leak at Compound 19 led to the deaths of at least 105 people, the Soviets finally decided to get rid of their cache. Huge vats of anthrax spores were mixed with bleach and transported the port town of Aralsk, on the shores of the Aral Sea (now 16 miles (25km) inland), where they were loaded onto barges and transported to Vozrozhdeniya. Some 100 to 200 tonnes of anthrax slurry was hastily dumped in pits and forgotten.

Most of the time, anthrax bacteria live as spores, an inactive form with extreme survival skills. They’ll shrug off pretty much anything you care to throw at them – from baths of noxious disinfectants to being roasted for up to two minutes at 180C (356F).

When they’re buried in the ground, the spores can survive for hundreds of years. In one case, they were recovered from an archaeological dig at the ruins of a medieval hospital in Scotland – along with the several-hundred-years-old remains of the lime they tried to kill them with.

More recently, a 12-year-old-boy died after being overcome by anthrax that had been lurking in the far north of Russia. The outbreak hospitalised 72 people from the nomadic Nenets tribe, including 41 children, and thousands of reindeer perished. It’s thought to have started when a heatwave thawed the carcass of a reindeer that was at least 75 years old.

As you might expect, the Soviets’ efforts at Vozrozhdeniya weren’t nearly enough. Years after the USSR’s collapse, in the wake of attacks in Tokyo and revelations about an extensive bioweapons programme in Iraq, fears were mounting about the prospect of terrorists or rogue governments getting their hands on any weaponised pathogens. So the US government sent teams of specialists to do some tests.

The precise location of the anthrax cache was never disclosed, but as it turns out this wasn’t a problem. The pits were so enormous, they were clearly visible in photos taken from space. Viable spores were found in several soil samples, and the US pledged $6m (£4.6m) for a project to clean the place up.

This involved a deep trench, dug next to the pits, some plastic lining and thousands of kilograms of powerful powdered bleach. All the team had to do was move several tonnes of contaminated soil into the trench – in 50C (122F) heat, while wearing full protective suits. In all, 100 local workers were hired and the project took four months to complete.

Just look for a bloated carcass with haemorrhaging – Les Baillie

It worked. After stewing for six days with the powdered bleach, the spores were gone.

But that’s not quite the end of the story. Half a century of open-air testing has left the entire island contaminated – not just at the test site, but all over. “Oh, there will still be anthrax there, no problem,” says Les Baillie, an international expert on anthrax from Cardiff University. He spent a decade working at the UK’s former bioweapons research facility, Porton Down.

That’s not to mention the burial pits of infected animals, with up to a hundred corpses in each, or the unmarked grave of a woman who died while handling an infectious agent some decades ago. “Even when you bury an animal, you have to bury it a good couple of metres down. If the area floods the spores can float back up and earthworms in the soil can move it around,” he says.

Chillingly, there is a similar site much closer for comfort than the steppes of Central Asia: Gruinard, a small island just off the coast of the Scottish Highlands. From 1942 to 1943, just one year, it was the epicentre of the UK’s bioweapons programme. The tests involved tethering sheep in an open field or securing them in wooden frames, then exposing them to large doses of anthrax. Once it was exploded over the island, another time it was dropped from a plane.

The sheep would start dying three days later – “you can tell when an animal has died of anthrax. Just look for a bloated carcass with haemorrhaging,” says Baillie – after which their carcasses were carefully disposed of. The scientists burned the bodies and even dynamited a cliff over some to contain the contamination.

Just this single set of experiments rendered the island so contaminated, initial efforts to clean it up failed and the site was abandoned.

The only people to set foot there in half a century were scientists from Porton Down and two brothers, the Fletts, from the mainland. They rowed the 10-minute trip across the sea once a year to repaint the warning signs – and wore protective suits while doing so.

Soil samples taken in 1979 revealed that, nearly four decades later, there were still between 3,000 and 45,000 spores per gram of soil. Proposals for dealing with the “contaminated monster”, as it became known, ranged from concreting it all over, to removing the top layer of soil and dumping it in the North Atlantic.

In the end, every inch of the 1.96 sq km island was sprayed with 280 metric tonnes of formaldehyde solution mixed with seawater. It was finally declared safe in 1990. Today the island can be accessed easily by boat – though you’ll have to convince someone to take you first.

They took us to the front door of the place and said ‘we’ll wait outside’ – Dave Butler

Thankfully, Vozrozhdeniya is not quite so accessible. To get there, Middleton, Butler and their team travelled across Kazakhstan to Quilandy, a nearby village on the mainland. The plan was to hire a boat to take them across the Aral Sea, and some guides. Naturally, the locals weren’t exactly falling over themselves to visit the notorious island – “They knew to stay away,” says Middleton – and in the end, they made an unlikely alliance with a gang of salvage-seekers.

The trip was delayed, as crew members were struck down by food poisoning. Hours after they were set to leave, a massive dust storm broke out, engulfing the village and the Aral Sea. “It was like the end of the world. We would have been in the middle of the storm in these rickety boats,” says Butler. “I don’t think we would have survived.”

The next day, they finally made it. The base is divided into two parts: the town of Kantubek, which was built to house scientists and their families, and the lab complex, which lies about two miles (3.2km) further south.

“Even once we got there, there was quite a way to go,” says Butler. The team had arrived from Kazakhstan, due to the difficulty of getting a visa from Uzbekistan – though this is where the base is actually located.

They traversed the island’s desert interior by motorbike, navigating without maps – “I think they used the Sun,” says Butler – while dressed in full biocontainment suits.

Though they knew it was dangerous, the gang had made several visits to the town before, ripping out copper pipes, removing light fixtures, gradually dismantling the town and scavenging what they could sell. “When you first see it, it looks like they’re still building it,” says Middleton.

Today Kantubek is a dilapidated ghost town, in which the signs of a once-comfortable life contrast with hints of something altogether more menacing. On the one hand, there are houses, a canteen and a couple of schools; on the other, the cracked portraits of military personnel, books by Marx and Lenin, and rusting tanks. “It’s weird because there’s this eerie sense of decay, but then there are incongruous elements, like a big war mural of a cartoon duck by a child’s playground,” he says. “There isn’t a single bird or insect – it’s totally quiet.”

The local gang was keen to get off the island as quickly as possible, so the crew didn’t have long. Soon they set off again, this time in search of the lab complex. “They took us to the front door of the place and said ‘we’ll wait outside’. They didn’t want to go in,” says Butler.

It just looks like they trashed the place and left – Nick Middleton

What they found at the site – officially called the Field Scientific Research Laboratory, or PNIL in Russian – was extremely disturbing. “The research buildings aren’t cleaned up at all,” says Middleton. “It just looks like they trashed the place and left.”

Vast glass tanks of hazardous substances line the walls, while the floor is covered in hundreds of thousands of smashed glass vials, pipettes and petri dishes. Discarded full-body suits, complete with alien-like masks and air hoses, are everywhere. The whole place has the feel of a dystopian video game – partly because it is (it’s featured in a version of the first-person shooter Call of Duty).

Here Butler stepped the safety up a notch and the team donned more complete breathing apparatus that filters the air. “Buildings tend to concentrate whatever’s there,” says Butler. In addition to stray anthrax, the team ran the gauntlet of formaldehyde, which is carcinogenic if you breathe it in.

But the sense of control didn’t last long. “We’d been in there for about 15 minutes and the canisters started to become defeated,” says Butler. When an air filter is overloaded, the first sign is usually a whiff of some noxious aroma which has snuck through. “It can happen if you get a real corrosive, industrial chemical in concentrated quantities.”

Whatever it was, they decided to get out, fast. Butler was happy to camp overnight and visit the testing range the following day, but the others had seen enough. “For me it was quite exciting – a chance to put all the knowledge I have into practice,” says Butler. “But I suppose I’m weird like that.”

As an extra precaution, Butler took nasal swabs from every member of the team and checked them for anthrax spores.

He had good reason to be worried. There are several ways to die from anthrax, and the gruesome details of each depend on how you were infected. There’s the gastrointestinal route, which is common in grass-eating animals such as cattle, horses, sheep and goats and still leads to human deaths in developing countries to this day. The symptoms vary, but tend to include vomiting, diarrhoea, and lesions all the way from the mouth to the intestines.

Failing that, skin contact alone is often enough; back in 19th Century Yorkshire, so-called “woolsorters disease” was an occupational hazard for people who worked in the textile industry.

But by far the most unpleasant fate is to inhale some. Once a spore makes its way into the body, first it hitches a lift to the lymph nodes. There the spores begin to hatch and multiply – eventually spilling out into the bloodstream and leading to widespread tissue damage and internal bleeding. It’s thought that the whole process can take months to complete, but in the end, at least eight out of 10 people die in the process.

We had to make sure we didn’t have any spores in the, erm, hairy parts of our bodies – Dave Butler

“It’s probably an ideal biological weapon as is,” says Talima Pearson, a biologist from Northern Arizona University who helped to sequence the strain that caused the outbreak at Sverdlovsk. “They were probably getting it from out in the wild.”

And not all of it was ordinary anthrax. Aralsk-7 was built amidst a bioweapons arms race with the US and the UK – a perilous mission to take already-lethal pathogens and make them even more hardy, infectious and deadly. Pains were taken to ensure bacteria were resistant to antibiotics and viruses could infect even those who had been vaccinated.

To achieve this, the scientists grew up industrial quantities of pathogens collected from the wild and honed in on those with the right characteristics. “The more material, the more chances there are to find what you’re looking for,” says Baillie.

But on 10 April, 1972, the three signed a treaty agreeing to give it up. This is precisely the moment that the Soviets launched the most terrifying programme yet. This time, they would use the emerging science of molecular genetics. These bioweapons would be designed, not just cultivated.

This included a particularly nasty strain of anthrax, known to researchers as STI. For starters, it was resistant to an impressive array of antibiotics, including penicillin, rifampin, tetracycline, chloramphenicol, macrolides and lincomycin. But that’s not the only reason you really, really don’t want to be infected by STI.

As if regular anthrax wasn’t bad enough, the scientists decided this natural killer needed a final flourish: toxins which can rupture red blood cells and rot human tissue. Scientists took the genes from a close relative, Bacillus cereus, and added them using the latest scientific techniques.

Anthrax naturally grows in clumps, but these can get caught up in the nostrils and don’t always lead to an infection. So the Soviets liked to grind them down using industrial machinery. The final result is just five micrometres long – at least 30 times smaller than the width of a human hair. “That’s the perfect size to be inhaled,” says Butler.

Before the team left for the island, Butler constructed a decontamination zone on the beach – basically just an outdoor tap – and stockpiled antibacterial soap. When they returned, every member stripped down naked and scrubbed themselves clean. “We had to make sure we didn’t have any spores in the, erm, hairy parts of our bodies,” he says.

Thankfully the team’s swabs came back negative and even the salvage-seekers, who refused their offer of protective gear, escaped unscathed. For the moment, the anthrax at Vozrozhdeniya remains in the ground.

This was a highly virulent strain, first isolated from an Indian man who brought it to Moscow in 1967

But what of the mysterious outbreaks in the 1970s and 80s? It’s now known that the Lev Berg strayed into an aerosol cloud of weaponised smallpox that had recently been exploded on the island. The incident was suppressed by the Soviet powers of the time, including KGB boss Yuri Andropov who later became Soviet premier. It’s not known exactly which strain they were infected with, but according to David Evans, a virologist at the University of Alberta, Canada, it’s likely to have been India-1967.

“We know this because this is the strain the Soviets sequenced,” says Evans. “They used a very old fashioned method which required astonishing quantities of DNA to do it, so it makes sense that they’d sequence the same one that they were weaponising.”

This was a highly virulent strain, first isolated from an Indian man who brought it to Moscow in 1967. There are two possible reasons it was able to infect those who had already been vaccinated: the vaccination didn’t work, or they were exposed to a particularly high dose.

“The Soviet vaccine was criticised, so it’s possible it just wasn’t working very well,” says Evans. “And a very high dose of anything can overcome an immunisation.” If the vaccine wasn’t working, India-1967 would have been a particularly dangerous virus to be exposed to.

So could the island still be infectious today? “Oh it would be long gone,” says Evans. The Russians recently rediscovered the victims of a smallpox epidemic in Siberia, after melting permafrost exposed their graves. Though their corpses had been frozen solid for 120 years, the scientists didn’t find any virus – just its DNA.

Evans works on the vaccine strain of the virus, which is related but only causes a localised skin infection. “Even in my lab where we store it in a -80C (-112F) freezer under ideal conditions, the virus slowly loses infectivity over time,” he says.

As for the plague, though the Soviets were working on weaponising it, the bacteria remains widespread in Central Asia to this day – in fact, the number of cases increased sharply after the USSR collapsed. Which just leaves us with the fish and the antelope. Both remain a mystery, but the widespread pollution in the Aral Sea at the time and more recent mass antelope die-offs suggest that both had alternative causes.